A polybenzazole fiber has superior heat resistance and flame resistance, as well as sufficient strength and elastic modulus, so that it is expected to make a super fiber of the next generation. The process for manufacture of the polybenzazole fiber is described in, for example, Japanese Patent Application under PCT laid-open under Kohyo No. 500529/1988, wherein a dope containing a polyphosphoric acid solvent is cooled and solidified to give a dope filament, which is brought into contact with water or an aqueous solution of polyphosphoric acid contained in the dope to allow coagulation, and washed with water, followed by drying to afford a product. The polybenzazole fiber after drying is heat-treated to give a polybenzazole fiber having a high elastic modulus. When an industrial scale production of polybenzazole fiber is desired, a step of stretching a dope filament, a step of extracting a solvent from a dope filament, and a step of evaporating water from a fiber containing a large amount of water need to be finished in a short time. Japanese Patent Unexamined Publication Nos. 157918/1995 and 157919/1995 disclose spinning; and Japanese Patent Unexamined Publication No. 197307/1995 discloses a technique of drying to realize continuous production at a spinning rate of 200 m/min or above. Although the fibers manufactured by such steps have superior dynamic properties, such as strength and elastic modulus, as compared to fibers produced by spinning at a low rate of not more than 80 m/min and washing with water and drying under a low tension, the polymers constituting the former fibers become dense, which in turn slows down markedly the release of water contained inside. In addition, internal distortion tends to occur due to the surface tension of water filled in very narrow holes of about 20 .ANG. within the fiber. A polybenzazole fiber having a high water content may have a decreased strength due to hydrolysis in the fiber when it is exposed to high temperature environment.
It is therefore an object of the present invention to provide a polybenzazole fiber having high strength and high elastic modulus, and which hardly suffers from occurrence of internal defect or hydrolysis, as a result of quick release of water during elevation of temperature for quick heating of the fiber.